1. Field of the Art
The present invention relates in general to a method and apparatus for producing a resin-coated metal part, and more particularly to a method and apparatus adapted to apply a resin layer to an outer surface of a metallic core member to produce a resin-coated metal part, while the core member heated to an elevated temperature is placed within a powdered mass of a thermally fusible resin.
2. Related Art Statement
Various resin-coated metal parts are known. For example, a metallic core member of a rotor for a rotary fluid machine of a Roots type such as a supercharger for a motor vehicle is coated at its outer peripheral surface with a thermally fusible synthetic resin. For applying such a synthetic resin coating (hereinafter called "resin layer") to the outer surface of a metallic core member, various methods are known. For instance, the metallic core member is first heated and then embedded within a powdered mass of a synthetic resin accommodated in a suitable container, so that a portion of the powdered mass surrounding the outer surface of the core member is melted and deposited on the outer surface of the core member. Thus, the outer surface of the metallic core member is coated with a resin layer. In one form of the above coating method, a gaseous fluid is blown into the powdered mass through a gas-permeable bottom wall of the container, to maintain the powdered mass in a fluid state and thereby enable the heated metallic core member to be easily immersed in the powdered mass. The above method (hereinafter referred to as the "powdered coating method") wherein a synthetic resin powdered is applied to a heated metallic core member, permits formation of a resin layer of a relatively large thickness in one coating cycle, with comparatively less costly equipment, and is therefore widely utilized in the industry to produce resin-coated parts.
The powdered coating method is effectively practiced where the synthetic resin used is polyethylene, nylon 11, 12 or other material which has a relatively low melting point and a relatively high thermal decomposition point. However, where there is a comparatively small difference between the melting and thermal decomposition points of a synthetic resin, for example, where a resin containing fluoroethylene is used, the powdered coating method is not satisfactory in terms of the thickness of a resin layer that can be obtained in one coating cycle (by one placement of a heated metallic core member into a powdered mass). In the powdered coating method, the fusion of a synthetic resin and consequent deposition thereof to the surface of the core member occurs after the heated metallic core member has been embedded or immersed in the powdered mass, and before the core member has been cooled below its melting point. Therefore, the smaller the difference between the melting and thermal decomposition points of the synthetic resin, the shorter the period during which the fusion of the synthetic resin occurs. Accordingly, where the above-indicated difference of the resin material is small, the thickness of the resin layer to be formed on the outer surface of the core member in one coating cycle is not sufficient. To obtain a sufficient thickness of the resin layer, the coating cycle must be repeated several times. On the other hand, heating of the metallic core member must be accomplished outside the powdered mass in which the heated core member is coated with a resin material of the powdered mass. Namely, these two steps of the coating cycle must be conducted at different locations. This requirement further complicates the powdered coating process, and requires a relatively long time to complete the process.